The present invention relates to compound transmissions having a multiple speed ratio main transmission section connected in series with a multiple speed ratio auxiliary transmission section, and more particularly, to such transmissions having shift rail interlock mechanisms.
Compound mechanical change gear transmissions comprising manually shifted multiple speed ratio main transmission sections connected in series with one or more auxiliary sections, usually of the range, splitter, or combined range/splitter type are well known in the prior art. The auxiliary sections are usually input and/or output auxiliary transmission sections, but within the scope of the present invention, may also comprise other devices, such as transfer cases or multiple speed drive axles. Examples of such compound transmission systems may be seen by reference to U.S. Pat. Nos. 4,754,665 and 4,944,197, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
As is well-known to those skilled in the art, it is necessary to provide some sort of interlock mechanism between the main and auxiliary transmission sections, to retain the main transmission section in neutral during a range auxiliary section shift transient and/or to prevent/inhibit shifting of the auxiliary section when the main section is not in neutral. An example of a range interlock mechanism is illustrated and described in above-incorporated U.S. Pat. No. 4,944,197, and also in U.S. Pat. No. 4,445,393, assigned to the assignee of the present invention and incorporated herein by reference.
Although the prior art interlock mechanisms have operated generally satisfactory in regard to the basic interlock function, there have been certain operating conditions in which the known interlock mechanisms have not been totally satisfactory. By way of example, in a compound transmission in which the auxiliary section is of the combined range/splitter type, a problem arises when the operator shuts down the vehicle with the transmission in a relatively high gear (for example, sixteenth gear on a sixteen forward speed transmission). This necessarily requires that the range/splitter auxiliary section be in its "high/high" condition. Later (for example, the next morning), when the operator resumes operation of the vehicle and tries to shift the transmission to a lower gear it may be possible to shift the transmission "down" only as far as ninth gear (i.e., the lowest gear in the "high" range ), because the interlock mechanism prevents a range shift (i.e., from the high range to the low range). Typically, the range shift cannot be made without rotation of the main shaft, which will cause unblocking of the synchronizer in the auxiliary section, to permit shifting of the auxiliary section from the high range to the low range. This is especially true after a shutdown period when the oil in the transmission is cold and thick, and there is a tendency for the synchronizer to get "hung up" on the low blocker.